Recent receptive field (RF) mapping studies identified a honeycomb-like mosaic of discrete place-defined columns - segregates - in the forelimb and orofacial regions of cat and monkey SI. The segregate topographic organization has fundamental implications for how SI responds to tactile stimulation. Specifically, it predicts that (1) a tactile stimulus produces a spatial pattern of active neuronal populations that correspond to individual segregates; (2) each active segregate develops a stimulus- specific alterating pattern of active/inactive minicolumns; and (3) even a point tactile stimulus evokes activity in a large number of segregates, some of which are activated first (directly from the thalamus), while others are activated later and less strongly via neighboring segregates. The purpose of the proposed study is to test these predictions experimentally. Three experimental series are designed to provide information on how stimulus-evoked activity in SI changes as a function of: (1) horizontal position of the recording site (minicolumnar, segregate position); (2) laminar position of the recording site; and (3) stimulus position on the skin relative to the segregate RF center. To this end, single unit (SU) data will be collected in tangential and radial microelectrode penetrations of cat area 3b; responses of SUs to precisely controlled punctate stimuli will be measured for stimuli placed at the RF center of the host segregate, at the RF center of an adjacent segregate, or in a linear array of sites away from the segregate RF center. The latency and magnitude of stimulus-evoked SU activity will be statistically analyzed to determine whether (1) an active segregate contains clusters of very similarly responding neurons organized in radially oriented ca. 40 micro(m) diameter minicolumns; (2) adjacent minicolumns have very different levels of response, with these differences exceeding the average difference among minicolumns within the entire segregate; and (3) stimulus-evoked activity within a segregate shows no overall change across the segregate, but changes abruptly to a new overall level at the border between segregates. The SU data will also be used to estimate the extent of the skin area that provides input to a typical segregate, and how fast the latency and magnitude of stimulus-evoked activity changes in different layers of a segregate with stimulus distance from the segregate RF center. the information will be used to reconstruct the spatio-temporal sequence of activity spread through layers in the activated field of segregates. The proposed study is expected to advance the understanding of spatio-temporal organization of SI response to peripheral stimuli; an essential step towards understanding SI functional operation and information processing.